1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor having high durability and producing high-quality images for long periods, and more particularly to an electrophotographic photoreceptor having good surfaceness and cleanability. In addition, the present invention relates to an image forming apparatus and a process cartridge therefor using the photoreceptor.
2. Discussion of the Related Art
Recently, a photoreceptor having an organic photosensitive layer including an organic photoconductive material on a substrate has typically been used as an electrophotographic photoreceptor (hereinafter referred to as a “photoreceptor” or an “image bearer” as well) installed in laser printers and digital copiers. Particularly, a multi layer organic photoreceptor having individual layers including a charge generation material (CGM) and a charge transport material (CTM), respectively has mostly been used because of its cost, productivity and material design flexibility, etc.
Since the electrophotographic photoreceptors are exposed to a mechanical external force and an electrical or a chemical hazard, they have various deteriorations in electrophotographic image forming process. Particularly, the electrophotographic organic photoreceptors repeatedly used for long periods to produce full-color images which have been more produced recently are required to have durability against the deteriorations more than ever.
The photoreceptor is abraded when a toner mainly formed of a colorant and a resin, which includes a hard inorganic particulate material as an additive is used, strongly pressed to a paper including a hard fiber or a clay when transferring a toner image onto the paper, or strongly frictionized by a cleaning blade when cleaning the photoreceptor. Therefore, Japanese Patents Nos. 2520270 and 3585197 disclose using polycarbonate or polyarylate having high durability as a binder resin in an organic photoreceptor.
Various photoreceptors including protection layers on the surfaces have been suggested to improve mechanical durability thereof. A protection layer including a dispersed hard particulate metal oxide is disclosed or a crosslinked protection layer is disclosed in Japanese published unexamined application No. 2001-166521. Japanese published examined application No. 6-82221 and Japanese published unexamined application No. 2002-1966523 disclose a method of lubricating the surface of a photoreceptor to prevent the surface abrasion, and which is in practical use.
As mentioned above, the photoreceptors producing more full-color images are required to produce images having higher quality than ever. Particularly, cleanability to remove a toner remaining on a photoreceptor every time when transferring a toner image onto a transfer material is required to improve more than ever. Photoreceptors having some abrasion resistance have some cleanability, but not yet satisfactory. Photoreceptors occasionally have small scratches and cracks on the surfaces when repeatedly producing images for long periods, and a toner occasionally scrapes off from a chipped blade. Particularly when a toner scrapes off from a chipped blade, images have black stripes and quality thereof deteriorates at once. Image deterioration due to a scraped-off toner needs an urgent solution for a highly-durable photoreceptor.
The surface of a photoreceptor is mostly abraded by a cleaning blade, however, when the transferability of a toner is increased, the toner remaining on the photoreceptor decreases and a pressure to the cleaning blade decreases, which prevents the blade from being chipped. Japanese published unexamined application No. 2001-66814 discloses a method of forming concavities and convexities having a specific shape such as prism, wave, cone, pyramid and well shapes on the surface of a photoreceptor with a touch roll or a stamper to increase the releasability of a toner. In addition, it is disclosed that a particulate filler including silicon or fluorine is included in a charge transport layer to improve transferability of a toner and reduce stress to a photoreceptor. However, the photoreceptor having concavities and convexities occasionally chips the edge of a cleaning blade when the surface resistivity of the photoreceptor becomes large in repeated image formation, resulting in scraping off of a toner.
Japanese published unexamined application No. 2007-233356 has recently disclosed a method of transferring a microscopic concave and convex shape on the surface of a photoreceptor. A reproducible concave shape is formed on the outermost layer with a mold while maintaining a specific relationship among a glass transition temperature of the charge transport layer, and temperatures of the mold and the substrate. Japanese Patent No. 3963473 discloses a method of roughening the surface of a photoreceptor by irradiating the outermost layer thereof with a laser beam to form plural concavities on the outermost layer. The outermost layer may include a resistivity controlling agent, but a residue thereof caused by irradiation of the laser beam remains in the concavities and is not removed with ease, resulting in hindrance to image formation.
Japanese published unexamined application No. 2005-99688 discloses a method of forming plural specific concavities on the surface of a photoreceptor to improve cleanability thereof. The concavities have openings having a specific long axis diameter, a minor axis diameter and a depth, and are present on the outermost layer at a specific surface density to prevent scratches thereon caused by external forces given by a cleaning blade, etc. in image forming processes. However, when an additive of a developer stays in the concavities and images are repeatedly formed, small black spots occasionally appear on the images. Conventional formation of concavities by a laser or a mold on the surface of a photoreceptor exactly form microscopic holes at high density and mechanically makes the outermost layer vulnerable.
Japanese published unexamined application No. 2005-99688 discloses a photoreceptor including a specific hardening resin as a binder resin and an organic or inorganic filler at the surface. Specific examples thereof include particulate carbon, particulate fluorine resins, particulate silicone resins, diamond-shaped carbon or inorganic fillers such as silicon oxides and titanium oxides. The hardening resin has an effect of preventing the filler from being dug up because of having more abrasion resistance than conventional resin matrices. However, when the outermost layer includes many particulate materials, the outermost layer is occasionally colored, resulting in insufficient light transmittance. Further, the organic filler which is soft causes insufficient durability. The hard inorganic filler is locally a large resistance to an elastic cleaning blade when projecting from the surface and chips the blade, resulting in occasional defective cleaning. So long as the outermost layer includes a filler even though the content thereof is small, the defective cleaning cannot totally be eliminated. Namely, the filler needs improving.
Japanese published unexamined application No. 7-92697 discloses a method of exerting a same effect of a filler without the filler. After an outermost surface is formed on an organic photoreceptor, a coating liquid for forming the outermost surface is diluted and sprayed with a spray gun on the surface to form convexities and concavities having a specific curvature thereon. Reversal of the cleaning blade and chipped edge thereof are prevented, but mechanical durability of the convexities needs improving.
Because of these reasons, a need exists for an electrophotographic photoreceptor having good cleanability, and preventing its cleaning blade from vibrating, kinking and reversing, and a toner from scraping off from the cleaning blade to cause defective cleaning and the toner having scraped off from fusion-bonding to the photoreceptor due to repeated image formation.